The present invention relates to a semiconductor integrated circuit having a power supply circuit and to a method of operating the semiconductor integrated circuit. More specifically, the present invention relates to a technology that is effective not only for reducing voltage drop when the power supply circuit makes a selection between a power supply voltage from a main power supply and a backup power supply voltage from an auxiliary power supply, but also for reducing the power consumption of the auxiliary power supply.
It is conventionally known that a backup power supply voltage from an auxiliary power supply is supplied to an electronic device in order to maintain the supply of power to the electronic device even in the event of a decrease in a power supply voltage from a main power supply that supplies electrical power to the electronic device.
According to Japanese Patent Laid-open No. Hei 11 (1999)-252825, a power supply voltage from a main power supply formed of an automotive battery is supplied to an electronic device through a first backflow prevention diode and a backup power supply voltage from an auxiliary power supply formed of an auxiliary battery is supplied to the electronic device through a second backflow prevention diode. The power supply voltage from the main power supply formed of the automotive battery is supplied to the anode of the first backflow prevention diode. The backup power supply voltage from the auxiliary power supply formed of the auxiliary battery is supplied to the anode of the second backflow prevention diode. The cathode of the first backflow prevention diode and the cathode of the second backflow prevention diode are commonly coupled and further coupled to the electronic device.
According to Japanese Patent Laid-open No. Hei 5 (1993)-276688, a power supply voltage from a main power supply formed of an AC adapter is supplied to a DC-to-DC converter and a load through a backflow prevention diode and a power switch, and a backup power supply voltage from an auxiliary power supply formed of a secondary battery is supplied to the DC-to-DC converter and the load through a MOS transistor and the power switch. As a charge circuit is coupled between the auxiliary power supply and the main power supply formed of the AC adapter, the secondary battery of the auxiliary power supply is charged by the charge circuit. The gate of the MOS transistor coupled to the output terminal of a comparator. A voltage obtained when the power supply voltage from the main power supply is divided by two voltage divider resistors is supplied to the non-inverting input terminal of the comparator. A reference voltage, which is generated by a reference voltage circuit formed of a resistor and a Zener diode from a power supply voltage supplied to the DC-to-DC converter and the load through the backflow prevention diode and the power switch, is supplied to the inverting input terminal of the comparator. Therefore, if the AC adapter is shut off from an AC power supply, the backflow prevention diode turns off. Thus, the divided voltage derived from the two voltage divider resistors becomes lower than the reference voltage of the reference voltage circuit. This turns on the MOS transistor. As a result, the secondary battery supplies a DC voltage to the DC-to-DC converter and the load through the drain-source path of the MOS transistor.